Coating of objects of magnesium or the like with oxides



- finishing treatment mediate treatment in baths composed according STATES PATENT aeessze COATING F OBJECTS OF MAGNESIUM 0R THE LIKE WITH OXIDES Ernst Krause and Ernst Schriider,

Leipzig, Germany; vested in the Alien Property Custodian No Drawing. rial N o. 1937 2 Claims.

This invention relates to the production or surface layers of oxide on objects made of magnesium or oi alloys consisting principally of magnesium by anodic treatment in alkaline solution.

As compared with the process hitherto known the process according to the invention differs essentially in that the anodic treatment is carried out in aqueous solutions of alkali salts of such metal oxides as have an amphoteric character, e. g. in solutions or alkali aluminates, arsenates or zincates. The protective layers obtained from such solutions are distinguished by high resistance to corrosion, great hardness and density, and good insulating power.

Preferably the alkalinity of the solution is maintained at a pH number oi about to 13 and the solution is kept at an elevated temperature, e. g. at a bath temperature of 30 to 70 C.

Since on long use of the bath due to changing the objects suspended therein, and due to carbonate formation the alkalinity oi the bath has a tendency to fall, the desiredhlgh alkalinity must if necessary be maintained by the addition of tree alkali. 1

The 'anodic' oxidation can be assisted according to the invention, by the addition of oxygen carriers, e. g. by the addition of small amounts oi cerium sulphate, or 01' tungstates or chromates.

It has been found that the protective layers of oxide which are obtainedby anodic treatment in baths of known composition can be considerably improved' as regards density, hardness, insulating power and resistance to corrosion by a or a preliminary or interto the invention. For obtaining high insulating power the treatment in such baths as contain alkali arsenates has been found to be particularly advantageous while moreover solutions of alkali aluminates produce specially favourable results. The solutions of these salts ofi'er also the special advantage that the metal (aluminium) is not cathodically deposited.

The conductivity oi. the baths is so regulated by suitable regulation or the concentration and temperature that the process is worked with currents of 1 to 3 amps. per sq. dm. By a large increase in the bath temperature e g. to 60 to 85 C. there are obtained dark or deep black coloured protective layers.

This colouration is probably to be ascribed to certain foreign metals in the magnesium or magnesium alloys. Instead of this, the protective layers obtained according to the invention can subsequently be coloured with organic or inorganic dyestuil's. or coated with lacquers.

The protective layers can also be sealed and hardened by treatment with water glass'and subsequent heating to about 150 C.

Appliczsttion November 5, 1938, Se- 238,976. In Germany November 18,

As examples of suitable bath compositions may be mentioned:

Example 1 Water litre 1 Sodium aluminate "grams.- 200 Cerium sulphate do 3 if desired acid or alkali additions are made thereto until the desired pH value is obtained.

if desired alkali or acid is added thereto until the'desired pH value is obtained. Temperature 50 C.

The process can be carried out using direct or alternating current. Whilst hitherto in the superficial oxidation of magnesium and magnesium containing alloys, alternating current had to be used, an advantage of the new process consists in that sufliciently strong protective layers can be obtained by the use of direct current under relatively low voltages e. g. 10 volts and with current densities which are about as high as those usual in the anodic oxidation of aluminium in aqueous solutions.

The protective layers obtained according to the new process produce even on profiled objects a good cormsion resistance and possess a high absorptive power for sealing agents such as water glass.

on account 01 the capability of the protective coating or being produced in thick layers the corrosion protection is especially good since thick layers are capable oi absorbingcorrespondingly more sealing agent.

We claim:

1. A process for the production of protective layers of oxide on objects made of magnesium or magnesium alloys comprising making said objects the anode in a bath containing an alkali a'rsenate, and passing an electricv current through said bath.

2. A process for the production of protective oxide layers on objects made of magnesium or bath, while keeping the temperature of the bath at a temperature or from 30 to C.

ERNST KRAUSE. ERNST SCHRbDER. 

